Impact of iceberg scouring on polar benthic habitats. Julian Gutt* ... resulting in parallel gouge marks < 1 m deep (Fig, 2a) .... competition for space and food.
MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser
Vol. 137: 311-316, 1996
Published June 27
NOTE
Impact of iceberg scouring on polar benthic habitats Julian Gutt* Andreas Starmans, Gerhard Dieckmann Alfred Wegener Institute for Polar and Marine Research, ColumbusstraBe, D-27568 Bremerhaven, Germany
ABSTRACT In s j t u photographs and videos demonstrate that iceberg grounding in both polar regions causes considerable damage to benthic communities. Sessile organisms are eradicated and pioneer species begin to grow in high abundances on the devastated substratum. A preliminary quantitative analysis shows that the sea floor in the Antarctic and Arctic areas of investigation is disturbed by icebergs statistically once every 230 and 53 yr, respectively. Due to the extreme slow growth of many species, particularly in Antarctica, areas frequently disturbed in this manner are likely to be characterised by a continuous natural fluctuation between destruction and recovery. Increased perturbation by iceberg groundings through predicted global warming will result in considerable impairment of this environment. KEY WORDS: Antarctic. Arctic Benthos . Iceberg scouring Global warming
Recently, the calving of Antarctic ice shelves has been discussed with respect to global warming (Doake & Vaughan 1991, Gammie 1995) and the consequences
of resulting iceberg scouring on the structure of the sea bed have been documented for both polar areas (Lien et al. 1989, Woodworth-Lynas et al. 1991). Yet the possible effect on the underlying benthic communities, other than by small growlers, has not yet been broached. Our analysis shows for the first time the impact of iceberg grounding and scouring on the Antarctic and Arctic benthos. The results provide an idea of the benthic system's resilience to such natural catastrophic events. They also enable us to assess consequences of a possible atmospheric warming, to which both polar ecosystems are considered particularly sensitive (Houghton et al. 1990). Material and methods. The sea floor was videotaped by a remotely operated vehicle and photographed in the Antarctic Weddell and Lazarev Seas (47OW to 12OE, 69's to 79's) and the Amundsen and Bellingshausen Seas (120° to 65OW, 67's to 73's; Fig. l a ) as well as off northeast Greenland (77ON to
Fig. 1. Areas of investigation: (a) Antarctic, (b) off northeast Greenland
0 Inter-Research 1996 Resale of full article not permitted
Mar Ecol Prog Ser 137:31 1-316,1996
Fig. 2. ( a ) Sediment surface honed by a horizontally drifting iceberg (off northeast Greenland, 7g011'N,15O09'W, 51 m). First pioneer organism was the motile pectinid bivalve Arctinula greenlandica. Width in the foreground: -80 cm. (b) Crumbling e d g e of a gouge mark (Amundsen Sea, 7 2 ' 5 0 ' S , 121' 16'W,384 m).Width in the foreground: - 100 cm
82'N, 6 O W to ly0W; Fig. l b ) between 1988 and 1993. Scour marks were found virtually at all depths on the Antarctic shelf (< 500 m), but only above 70 m in the Arctic. The abundances of all visible organisms were analysed. Results and discussion. Apparent irregularities in the benthic structure were identified as iceberg scour marks under 2 assumptions:
(1) The fine-scale bottom topography revealed an otherwise inexplicable mechanical disturbance, resulting in parallel gouge marks < 1 m deep (Fig, 2a) or resembling an irregularly ploughed field. Such marks were occasionally bordered by embankments which had begun to crumble (Fig. 2b). Here, as a consequence, all sessile organisms had been eradicated.
Gutt et a1 Impact of iceberg scouring
Fig 3 Underwater video strip transect (Antarctic, Wfddell Sea, 7 1 23'S,13 57'W, 307 m ) . M - \ l < indicate sections viith a mature and diverse species assemblage consisting mainly of sessile suspension feeders; dominant 'sessile/sedentary taxa': Thoiiarella s p (Gorgonaria), Abyssocucun~i'i l ~ o u v i l l n (Holothuroidea),S y n o i c m addreanurn [\Scidiacea), Astrotonia a q a s w i i (Ophiuroidea), Cephalodiscidae spp. (Pti'robranchia) Most abundant 'motil" taxa' Ponidchocrinus kt'rguelensis (Cl-inoidea), '\steroidea spp., ( ' I d a n d a e (Echinoidea). 'Sponges' df'mospongiae (e.g. Stylocordyla borealis, ('lnachyrd barbata) a n d hexactincllida ( e g. Roc;c;txlla racovitzae. R nuda, R antarctica) S a rocent scour mark which was almost devoid of benthos with the exception of a hoxdctinellid sponqu lying in a small depression R ' scour mark partly recolonised by motile species ( e , g the crinoid Poniachocrinus kerquelunsis) and by the bryozoan C'cllana sp. vi-ith a higher cover in most parts ot K than in M. M-)
0
100
200
Ophiuroidea sp
Me dian (n m J 2 i percentile (n m ') -. ¥ percentile ( n m ') Area units investigated
Within d a
Outside d a.
147 103 218 7 (= 5 7 my
59 48 74 58 (= 53 1 m )
1
39 Stylocoidyla borealis Re1 abunddnce ( n m ') 0 0-14 3 0 0-1 8 (stalked demosponge) Range (n m j Aiea units investigated 23 (- 12 7 m2) 53 (- 28 4 m2] Ldtruncuba apicahs (Ion~osponge)
2.3 Rel. abundance ( n n - ~ ' ) 0.0-13.3 0.0-11 Range (n ni - 1 Area units investigated 75 (= 65 9 m2) 77 (= 68 4 m?]
Rel abundance ( n P1c;ta sp (trrcbcllid polychaete) Range (n in J Median ( n m J 25' , percentile ( n m-2) 75 ";, percentile Area units investigated
16 6 4 3-221 0 0-30 19.8 11 5 22.1 6 (= 14 1 m2) 234 (- 684 8 m')
59 7 Young specimens of Rel abundance ( n m 2 ) Synoic~urnaddreanurn Range (n nl j 37 4-82 9 0 0-43 4 (compound ascidian) ~~d~~~ m 2 ) 61 7 113 40 4 25' percentile ( n m ') 75' percentile 74 4 Area units investigated 6 (= 14 1 m'] 234 (= 684 8 m')
. .
300
400
500
600
videotransect (m)
Table 1 Most common pioneer taxa which occur in much higher abundances within (Within d a ) the disturbed areas than outside (Outside d a ) (Weddell a n d Lazarev Seas, Antarctica) Foi some taxa only the total number of specimens calculated per unit area (= ielative abundance) within and/or outside the disturbed area a n d the range of abundance (in parentheses) d u e to the low abundances For the other datd sots the medians w e r e calculated with the 25 a n d 75"; percentiles, since the abunridnces were sufficiently high Taxa
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l
(2) Areas were classified as having been disturbed by icebergs if a n abrupt change in the epibenthic sti-ucture could not be explained by other physical or biological processes (Figs. 3 & 4a-c). In this case ridges gouged by icebergs had already eroded. An alternative explanation is the vertical settlement of a n iceberg caused by a rising a n d falling tide. First immigrants after such events were motile organisms such as fish, echinoderms, or bivalves (Fig. 2a). Initial recolonising sessile species were hydrozoans (stylasteridae),sabellid polychaetes, different compound ascidians, and bryozoans (e.g. Camptopljtes tricornis, Antarctica). Occasionally some benthic organisms occurred in higher densities within the disturbed areas than outside those areas (Table l ) , On the crumbling edges of gouge marks, locally high numbers of sea anemones and 1 demosponge species were observed in the Antarctic, while in the Arctic dense concentrations of an unidentified infaunal polychaete appeared. This mass occurrence of only a very few species can be ex-
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plained by their explosive growth or an especially successful recruitment combined with the lack of competition for space and food. In Antarctica, dense patches of sponges (e.g. Rossella racovitzae, R. nuda, Scolymastia joubini, Cinachyra barbata)),of which some are extremely slow growing, were frequently observed on small slopes. This can be interpreted as a late recolonisation of exceptionally deep gouge marks, possibly favoured by localised small-scale upwelling. Also, high concentrations of bryozoan debris and sponge spicule mats (Fig. 4b), the possible consequence of iceberg grounding, may constitute the substratum for specifically adapted benthic assemblages. Quantitative analysis shows the relevance of iceberg scouring for the benthos over a long time span (Table 2). Key organisms in Antarctica, particularly sponges (Dayton 1978), are slow growing (Clarke 1983, Arntz et al. 1994) and other species depend on
Mar Ecol Prog Ser 137: 311-316,1996
Fig. 4 . Photographic transect in the Wedde!! Sea (?1°06'S 11°39'W 194 n-C).
(a) Abundances of macrobenthic organisms along the transect; each bar represents a photograph covering an area of 0.56 m2 Dominant 'sessilelsedentary taxa': Synoicium addreanum, Didemnidae spp. (both Ascidiacea], Ascidiacea sp., Sabellidae sp. (Polychaeta). Most abundant 'motile tdxa': Ophiuroidea spp., Notocidaris sp. (Echinoidea), Pomachocrin~~.~ keryuclensis (Crinoidea), Pycnogonida spp. 'Sponges'' Monosynnga longispina, Cinachyra barbata, Tedania tantula, which were disrupted by a gouge mark between 200 and 230 m. Arrows indicate position of photographs ( b & c, opposite)
Table 2. Absolute and relative abundance of iceberq scours in the Antarctic and Arctic areas of investigation Antarctic Areas of investigation
Lazarev Sea Southeastern Weddell Sea Amundsen Sea Bellingshausen Sea Investigated depth range (m) 100-500 Totiil no of stations invi stigated Area photographed (m ) Area video-taped (m'] Stations with gouge marks Total no. of gouge marks found Maximum no. of gouge marks per station Estimated maximum age of the gouge marks (yr] Disturbed area (m2] Disturbed area (%) Period (yr] in which statistically the area is disturbed once
Arctic Northeast Greenland
3 0 additional stations below 500 m water depth have been investigated, however, without any indication of gouge marks '17 additional stations below 70 m water depth have been investigated, however, without any indication of gouge marks
these (Gutt & Starmans 1996). Consequently, a mature benthic community will theoretically not become established within a period of 230 yr in the Antarctic and 53 yr in the Arctic area of investigation. However, the grounding of icebergs is patchy with the probability being higher in shallow areas, where the richest species assemblages occur. This may be why we find relatively small glass sponges in areas of the Weddell and Lazarev Seas which are prone to scouring, while much larger individuals of the same species were found in shallow areas of the Ross Sea (Dayton 1972) which are not likely to be affected by icebergs. Based on this, the question arises whether the perpetual change between such destruction and recolonisation is one of the major causes for the development of the high between-habitat biodiversity, especially in Antarctica (Dayton et al. 1994).
Gutt et al.' Impact of iceberg scouring
Fig 4b Iceberg gouge mark not yet recolonised. The sediment indicated a mechanical disturbance a n d consisted of a high proportion of gravel and sponge spicule mats
Fig. 4c Area adjacent to the gouge mark showing a rich and diverse epibenthic assemblage which totally covered the sediment. Sponges' Rossella I-acovitzae, Scolyrnastia loubini, Isodictyd s p . , Cinachyra barbata, Monosyringa longispina, Tedania tantula, Bryozoa Cheilostomata sp (Bryozoa), Cellarinella sp , Austroflustra vulgaris, R < = i t o p ~ ~ _ /vv.loaric, /:3
nth?!-
